Quantum key distribution involves establishing a key between a sender (“Alice”) and a receiver (“Bob”) by using weak (e.g., 0.1 photon on average) optical signals transmitted over a “quantum channel.” The security of the key distribution is based on the quantum mechanical principle that any measurement of a quantum system in an unknown state will modify its state. As a consequence, an eavesdropper (“Eve”) that attempts to intercept or otherwise measure the quantum signal will introduce errors into the transmitted signals, thereby revealing her presence.
The general principles of quantum cryptography were first set forth by Bennett and Brassard in their article “Quantum Cryptography: Public key distribution and coin tossing,” Proceedings of the International Conference on Computers, Systems and Signal Processing, Bangalore, India, 1984, pp. 175-179 (IEEE, New York, 1984). Specific QKD systems are described in publications by C. H. Bennett et al entitled “Experimental Quantum Cryptography” and by C. H. Bennett entitled “Quantum Cryptography Using Any Two Non-Orthogonal States”, Phys. Rev. Lett. 68 3121 (1992).
The general process for performing QKD is described in the book by Bouwmeester et al., “The Physics of Quantum Information,” Springer-Verlag 2001, in Section 2.3, pages 27-33, and Section 2.6, pages 43-46, which two sections are hereby incorporated by reference as background information.
To ensure secure operation of the QKD system, system statistics such as the error rate need to be monitored to detect attempted eavesdropping and to ensure that the system itself is not generating errors. For example, a change in the error rate could indicate either the presence of an eavesdropper or a drift in the calibration of the system (e.g., the modulator voltage). Further, ensuring secure system operation includes setting up the QKD system to operate properly in the first place so that the ideal or optimum operation regime can be maintained by monitoring changes in system statistics from the ideal or optimum.
FIG. 1 is a plot of the number of photon counts vs. time for two single-photon detectors in a phase-encoding QKD system. The plot includes sections I-IV associated with respective modulator phase values. To set up the QKD system for ideal operation, there are a number (e.g., eight) modulator states that need to be adjusted. However, displaying these different states on the type of plot of FIG. 1 yields little visual information about the actual state of the system and the type or source of errors. In fact, the error rate statistic alone does not indicate the source of errors, and is not particularly sensitive to certain types of eavesdropping.
More generally, the prior art relating to QKD systems emphasizes operation of prototype systems in their ideal states but provides no discussion about how to actually set up a QKD system to run in the ideal state. Further, there is essentially no discussion in the prior art of how to efficiently monitor the unique QKD system statistics to quickly and efficiently diagnose system errors to start up the system up as quickly as possible and keep it running in its ideal state.
The various elements depicted in the drawings are merely representational and are not necessarily drawn to scale. Certain sections thereof may be exaggerated, while others may be minimized. The drawings are intended to illustrate various embodiments of the invention that can be understood and appropriately carried out by those of ordinary skill in the art.